1. Field of the Invention
The present invention relates to a method for making improved lithographic aluminium offset printing plates according to the silver complex diffusion transfer reversal process and to the photosensitive monosheet layer assemblage used for making such printing plates.
2. Description of the Prior Art
The principles of the silver complex diffusion transfer reversal process, briefly called DTR-process herein, have been described in e.g. U.S. Pat. No. 2,352,014.
In U.S. Pat. No. 3,424,580 a DTR-process has been described, according to which a positive image is made on an anodized aluminium surface that is free from traces of sulphate, said process comprising image-wise exposing a light-sensitive element comprising a paper support and a silver halide emulsion and then developing the resulting latent image in the presence of a silver halide solvent while said image-wise exposed silver halide emulsion layer is in close physical contact with said anodized aluminium surface. According to this process the light-sensitive element and the anodized aluminium are attached to each other by a plastic hinge. During the image-wise exposure the hinged assembly has to be opened to allow the exposure of the light-sensitive silver halide emulsion layer to take place and in a subsequent step to allow wetting of the anodized aluminium surface with developer (not of the still light-sensitive emulsion layer comprising a latent image). Next, the assembly has to be closed to allow the diffusion transfer to take place from the emulsion layer to the aluminium surface. The purpose of the described process is merely to form a silver image on an aluminium surface, the imaged aluminium not serving for printing purposes. The described process has the disadvantage that it requires the use of a two-sheet assembly, which is difficult in handling during exposure as well as during development.
A lithographic printing plate can be made according to the DTR-process. In U.S. Pat. No. 3,511,656 a method has been described for making a printing plate by emulsion layer, a silver-receptive stratum containing nuclei for precipitation of silver from diffusing water-soluble silver complexes, and a base sheet e.g. and aluminium foil, and applying an aqueous alkaline solution of a developing agent and silver halide solvent to the photo-exposed silver halide emulsion layer, reducing the exposed silver halide, allowing the unreduced silver halide or complexes formed thereof to diffuse from the unexposed areas of the silver halide emulsion layer to the silver-receptive stratum to produce from the unreduced silver halide or complexes formed thereof in conjunction with the nuclei a visible silver image in the silver-receptive stratum, said image being oleophilic ink-receptive, removing the photo-exposed silver halide emulsion layer from the surface of the silver-receptive stratum with warm water, wetting the imaged silver-receptive stratum with aqueous dampening liquid to wet out the non-imaged areas, coating the silver-receptive stratum with an ink, which wets out the imaged areas, and pressing the inked surface onto copy sheets for the transfer of the ink image thereto. It is possible also to dispense with the silver-receptive stratum containing nuclei so that the oleophilic ink-receptive image is formed directly on the base sheet e.g. an aluminium foil, the surface of which has been rendered hydrophilic previously by brushing, silicating, anodizing, etching, or the like. By treatment with a lacquer the oleophilicity of the silver image can be increased, if desired.
In U.S. Pat. No. 4,772,535 a light-sensitive lithographic printing plate material has been described, which material comprises a support e.g. a metal support, an optional subbing or antihalation layer or undercoat, a non-light-sensitive silver halide emulsion layer, a light-sensitive silver halide emulsion layer, and an image-receiving layer containing physical development nuclei. The material is exposed image-wise through the image-receiving layer and developed to form a diffusion transfer silver image in the image-receiving layer (not in the metal support). The imaged element thus obtained is used as such as a printing plate without separation of the now useless emulsion layers from the layer that contains the printing image.
According to EP-A 0,278,766 a lithographic printing plate precursor has been proposed, said precursor comprising a grained and anodized aluminium foil coated with a sol containing nuclei in a gelatin binder and--according to one embodiment--covered with a silver halide emulsion layer. Extensive experimentation with a said printing plate precursor has shown unfortunately that satisfactory printing results can only be obtained on the condition that after development of said precursor, the residual emulsion layer is removed by washing with hot water (50.degree. C.) and that the image plate is treated with a finisher comprising large amounts (20 g/l) of trypsin. The use of hot water has several disadvantages. The cost of hot water is high. Moreover, hot water dissolves the proteinic binder, usually gelatin, of the emulsion layer, thus causing decomposition of said layer so that a dirty black waste water comprising silver particles and dissolved silver salts is obtained, which upon cooling may clog filters and draining pipes. As for trypsin, this is a proteolytic enzyme that should be present in the finisher to degrade or decompose the proteinic binder that has adsorbed onto the silver grains precipitating on the aluminium foil during image formation. Substantial amounts of proteinic binder can indeed easily reach the silver grains owing to the fact that a silver-receptive stratum comprising gelatin and a gelatin silver halide emulsion layer have been coated directly on the aluminium foil. After oleophilization of said silver image the adsorbed gelatin, which is inherently hydrophilic, constitutes an undesired hydrophilic element in the master image so that prints having an insatisfactory quality are obtained. Moreover, said trypsin, which is essential to degrade the proteinic binder in the silver image, is extraordinarily expensive and is ecologically harmful as can be derived from i.a. Sigma Aldrich Library of Chemical Safety Data : MSD Book, 2,35553A,B,C and from Registry of Toxic Effects of Chemical Substances, YN507500.
In addition to the above disadvantages it has also been established that the gelatin present in substantial amounts in the nuclei-containing layer and in the emulsion of the lithographic printing plate precursor layer has a corrosive effect on the aluminium foil. The corrosive effect of gelatin on aluminium has indeed been described by J. H. Penn and G. A. W. Murray in Br. Corros. J., 1967, Vol.2, September, pages 193-4. Even though the corrosive influence of gelatin on the aluminium foil may be limited thanks to the presence of the anodization layer thereon, this protection is incomplete owing to random defects in the continuity of the anodization layer.
Owing to electromotive force existing between the aluminium and the silver metal in close contact therewith, corrosion can be caused in the silver image areas. In spite of the barrier effect of the anodized surface to this process corrosion can occur in various parts of the image corresponding to anodic faults. As a result, corrosion specks are formed, which appear as pin-holes surrounded by a region of weakly adhered silver. These corrosion defects, even though their effect may be reduced by treatment with corrosion inhibitors, lead to unsatisfactory printing quality.
Furthermore, it is generally known that aluminium ions have a hardening influence on gelatin (see e.g. the paragraph bridging pages 78 and 79 of "The Theory of the Photographic Process" 4th Ed., edited by T. H. James). Aluminium ions of the foil can indeed cause a hardening reaction in the gelatin layers so that removal of the emulsion layer gradually becomes more difficult.
Finally, as a result of the corrosive effect of gelatin on aluminium and the hardening reaction caused by aluminium in the gelatin layers, the shelf-life of the lithographic printing plate precursor is limited substantially.